This invention relates to an improvement of a paper-type filter element which is to be used for a filter device such as an air cleaner, oil filter or fuel filter for an engine of automobile.
As known, a conventional filter element used for such a filter device as mentioned above is formed of a long belt-shaped filter paper folded into an accordion shape by folding it reversedly in multiplicity at certain length, said filter paper being disposed between a pair of parallel doughnut-shaped end plates in such a manner as to form a hollow cylinder as a whole. That is, as shown in FIG. 1, many numbers of rectangular sections 2 continuously connected with each other through ridge portions 1 are arranged radially and disposed between a pair of parallel end plates 3 to form a hollow cylinder 4.
There are two ways for increasing or enlarging the filtering area of filter element of this type; one is to increase the outer diameter R1 of said hollow cylinder 4 and to decrease the inside diameter R2 thereof for increasing the areas of the respective sections 2, and the other is to increase the density of arrangement of the respective sections 2 for increasing the number of sections 2. If said diameters R1 and R2 should be restricted due to some factors, only the latter way is made available.
In this case, there is a limit to increase the density of arrangement of the sections since it is required to provide sufficient spaces between adjacent sections thereby to permit smooth flow of a fluid which is to be filtered. This renders the upper limit of the density in arrangement of the filter paper.
According to the conventional element as shown in FIG. 1, the respective sections 2 of the filter paper are arranged generally radially and are symmetric with respect to the center of the cylinder 4. Therefore, the distance P1 between the adjacent ridge portions 1,1 at the inner peripheral side is smaller than the distance P2 therebetween at the outer peripheral side. In other words, the density of the filter paper thus arranged is high at the inner peripheral side and low at the outer peripheral side.
Consequently, even if the filter paper is so arranged as to have a density of maximum permissible value at the inner peripheral side, a waste of space is formed at the outer peripheral side since the arrangement of the filter paper is rough at the outer peripheral side than that at the inner peripheral side.
In this way, according to the conventional element, since the difference of the density in arrangement of the filter paper is necessarily produced, even if the density should be increased to be maximum permissible value, the wasteful space formed in the wide range of its outer peripheral side could not be eliminated substantially.
The present invention is accomplished in view of the above.